Conventionally, as thermal printheads, a thick-film thermal printhead (See Patent Document 1 below) and a thin-film thermal printhead (See Patent Document 2 below) are known.
Patent document 1: JP-A-H11-314390
Patent Document 2: JP-A-H08-310024
FIGS. 9 and 10 show an example of prior art thick-film thermal printhead. The thermal printhead B1 includes an insulating substrate 101, a partial glaze layer 102, a common electrode 103, a plurality of individual electrodes 104, a resistor layer 105 and a protective layer 106. The common electrode 103 includes a plurality of comb teeth 103a. Each of the individual electrodes 104 includes a front end positioned between adjacent two comb teeth 103a and the opposite end connected to a drive IC (not shown). Both of the common electrode 103 and the individual electrodes 104 are formed by thick-film printing using gold resinate paste. The resistor layer 105, which is in the form of an elongated strip, is formed by thick-film printing so that the resistor layer partially covers the comb teeth 103a and the individual electrodes 104 alternately.
In printing an image using the thermal printhead B1, current is caused to flow between each of the selected individual electrodes 104 and the adjacent two comb teeth 103a, whereby the portion 105a (hatched portion in FIG. 9 ) of the resistor layer 105 sandwiched between the two comb teeth 103a is heated. As a result, a predetermined portion of heat sensitive paper or an ink ribbon is heated, whereby printing is performed.
FIGS. 11 and 12 show an example of prior art thin-film thermal printhead. The thermal printhead B2 includes an insulating substrate 111, a partial glaze layer 112, a common electrode 113 a plurality of individual electrodes 114, a resistor layer 115 and a protective layer 116. The resistor layer 115 is formed, by sputtering, as a thin film extending over the partial glaze layer 112 and the insulating substrate 111. The common electrode 113 including the comb teeth 113a and the individual electrodes 114 are provided by forming a conductive thin film of A1 on the resistor layer 115 by sputtering and then patterning the conductive thin film by etching using photolithography. The front end of each comb tooth 113a and the front end of the corresponding individual electrode 114 face and are spaced from each other, and the exposed portion of the resistor layer 115, which is sandwiched between the comb tooth 113a and the individual electrode 114, serves as a heating portion 115a. 
In performing printing using the thermal printhead B2, the drive IC (not shown) causes current to flow between each of the selected individual electrodes 114 and the comb tooth 113a facing the individual electrode, whereby the heating portion 115a of the resistor layer 115 is heated.
However, the prior art thermal printheads B1 and B2 shown in FIGS. 9-12 have the following drawbacks.
In the thick-film thermal printhead B1, since the resistor layer 105 is a thick film, the heat capacity of the resistor layer 105 itself is large. Therefore, when the ON/OFF switching speed of energization is increased, the corresponding heating and heat dissipation at a high speed is difficult. When the responsiveness to the heating and heat dissipation is not sufficient, problems such as trailing or a blur of a printing dot may be caused in high speed printing or high definition printing.
Further, the resistor layer 105 comprising a thick film projects largely upward as compared with the common electrode 103 and the individual electrodes 104. Therefore, during the printing, the protective layer 106 covering the resistor layer 105 is pressed against heat sensitive paper or an ink ribbon with a high pressing force, and the friction may cause sticking which may result in unstable sheet feeding or abnormal noise. The sticking is likely to occur particularly when the ink ribbon is heated to a high temperature due to the heating of the resistor layer 105 and the ink component is molten.
In the thin-film thermal printhead B2, the common electrode 113 and the individual electrodes 114 are provided by forming a conductive layer on the resistor layer 115 and then etching only the conductive layer into a predetermined pattern while leaving the resistor layer 115. To enable such etching, the conductive layer is often made of Al, for example. However, electrodes made of Al are inferior to electrodes made of e.g. Au in corrosion resistance. Therefore, in the long-term use, the common electrode 113 and the individual electrodes 114 may be chemically or electrically affected to be corroded, which may result in contact failure or disconnection. Therefore, the durability and reliability of the thermal printhead B2 is not sufficient.
Moreover, the common electrode 113, the individual electrodes 114, the resistor layer 115 and the protective layer 116 are formed by sputtering as thin laminated films. Generally, sputtering is performed in a vacuum chamber and requires processing time corresponding to the intended film thickness. To form the thin films one upon another, the sputtering operation need be performed repetitively. Therefore, it is difficult to shorten the operation time, which leads to low manufacturing efficiency.